1. Technical Field
The present disclosure generally relates to filters, and more particularly to a low-pass filter.
2. Description of Related Art
Conventionally, when a wireless network device operates at high power, harmonic components of high frequency are generated due to the nonlinear properties of the active components of the device, causing electromagnetic interference (EMI).
To address this, a filter is often used to suppress the harmonic components. Some manufacturers use a waveguide element, such as a microstrip, formed on a printed circuit board of the device.
Features of an ideal filter are signal attenuation of zero within a pass band, becoming infinite within a stop band, and transition as sharp as possible from the pass band to the stop band, providing the shortest possible distance between a transmission zero point and the stop band. In addition, increased transmission zero points improve performance of the filter in suppression of harmonic noise.
Referring to FIG. 4, a commonly used low-pass filter 40 is shown. The low-pass filter 40 includes an input portion 400, an output portion 420 aligned with the input portion 400, a high impedance transmission portion 440 electrically connected to the input 400 and the output 420, a rectangular first low impedance transmission portion 460 electrically connected to the high impedance transmission portion 440, and a rectangular second low impedance transmission portion 480 parallel to the first low impedance transmission portion 460 and electrically connected to the high impedance transmission portion 440. An overall length of the low-pass filter 40 is 8.69 millimeters (mm), and an overall width of the low-pass filter 40 is 3.53 mm. An area of the low-pass filter 40 is 30.67 mm2.
FIG. 5 is a diagram showing a relationship between amplitude of insertion or return loss and frequency of an electromagnetic signal traveling through the low-pass filter 40. As shown in FIG. 5, only one transmission zero point is generated, therefore the low-pass filter 40 is not effective in the suppression of harmonic noise.
Therefore, a need exists in the industry to overcome the described limitations.